Recent developments in the technology relating to telecommunications and optical fibres make it possible to transmit signals over longer and longer distances without regeneration or amplification. In the present state of the art (mid 1984) it is possible to achieve distances of up to 140km without regeneration. This range is enough for inter-island traffic, e.g. from the south west of England to the Channel Isles. It is envisaged that future improvements in the technology will increase the range. The achievement of practical ranges without the need for regenerators makes possible fundamental changes in submarine cable technology and this invention relates a fundamentally new structure which is particularly appropriate for submarine telecommunications cables.
According to this invention, a submarine telecommunications cable comprises a plurality of optical fibres contained in the bore of a tubular strength member made of longitudinally-orientated thermoplastic polymer, e.g. polyethylene, wherein the fibres are embedded in an embedding medium which fills the space in the bore not occupied by the fibres. The embedding medium may serve a plurality of functions as described below.
It is intended that the whole cable shall be at substantially ambient pressure, i.e., at atmospheric pressure before it is laid and at hydrostatic pressure when it is on the sea bed. This permits the use of a less massive structure, but it is necessary to avoid voids which would collapse under the hydrostatic pressure.
It is preferred that each fibre is embedded directly in the embedding medium, preferably an elastomeric solid, e.g. a silicone elastomer. It is conventional to coat the surface of an optical fibre with a protective material, e.g. a silicone elastomer. The embedding medium also fulfills the function of these coatings and the materials used for the coatings are suitable the embedding medium. In addition, the embedding medium helps to locate the fibres in the centre of the bore.
The embedding medium also fulfills an emergency function in case the cable is accidentally damaged while submerged. If the damage permits access of the sea to the bore then the embedding medium prevents substantial entry of water and, especially, it prevents the water spreading for long distances.
The average density of the whole cable is an important aspect of the invention. Preferably the average density is in the range 0.9 to 4 g cm.sup.`2, e.g. 0.9 to 1.50 g cm.sup.-3.
Thus the invention differs from conventional technology, which utilizes massive cables, by the use of cables with low density, e.g. with substantially neutral buoyancy, e.g. average densities in the range 0.9 to 1.2 g cm.sup.-3. Recent developments provide means for burying cables in the sea bed and a buried cable will remain at the bottom even if it has positive buoyancy. Thus, where a cable has substantially neutral buoyancy, it is not important whether the residual buoyancy is positive or negative.
Cables with substantially neutral buoyancy have only a small weight in water which implies that they encounter loads substantially less than the loads of conventional (massive) cables. Breaking loads as low as 10 kg would be suitable for some applications, e.g. inland applications, but breaking loads of at least 100 kg, or preferably 500 kg, are needed for most submarine applications. On the other hand, it is considered that a breaking load of 3000 kg would be more than ample for almost all submarine applications, especially with non-massive cables. Breaking loads of 1000 kg would be adequate for most submarine applications and 2000 kg would be adequate even for those applications where high strengths are appropriate.
The tensile strength of the cable is substantially the strength of the strength member because the core element, i.e. the fibres and the embedding medium, make negligible contribution to the tensile strength.
A cable according to the invention may include extra components, i.e. components in addition to the optical fibres, the strength member and the embedding medium. Examples of extra components include, king filaments, claddings, and metal strands. Eachof these three extras will be separately described.
King filaments are often used in optical fibre cables to enhance the mechanical stability of the glass fibre package. A cable usually contains six optical fibres and these are arranged in contact with one another in a hexagonal pattern. To enhance the stability, especially during the assembly of the cable, the optical fibres are arranged around an additional filament usually called the "king filament" or, in the case of a metal filament, the "king wire". In cables according to the invention the king filament may be of a plastics material, e.g polyethylene, or preferably of glass, e.g. a seventh optical fibre.
Claddings are abrasion resistant external layers intended to protect the cable, especially during laying and handling. The claddings are conveniently of abrasion resistant thermoplastics for application by extrusion to completed cables. The use of a cladding reduces the incidence of failure because damage which is limited to the cladding has no effect on telecommunications performance.
Metal wires may be provided in case it is necessary to locate a cable on the sea bed. Location systems exist in which underwater electromagnetic detectors respond to electrical signals in conductive parts of the cable. When necessary, suitable signals are applied to the metal wires. Also, e.g. where cables according to the invention are used over distances greater than 40km, it might be desired to incorporate powered elements, e.g. regenerators and/or amplifiers, into the system. The metal wires mentioned above are suitable for providing electrical power where necessary.
Conveniently the metal wires are located between the strength member and the cladding. Although the purpose of the metal wires is to provide a path for electrical location signals or electrical power they have additional effects, e.g. increasing the tensile strength and increasing the average density. It is emphasized that any strands which may carry tension should be laid straight and not in a helical lay as is conventional in cable technology. The helical lay may cause unacceptable twisting in the cable (and conventional technology uses complicated torsionally balanced structures to avoid this). Straight strands do not cause twist but they may make only a small contribution to the strength.
Most surprisingly, a submarine cable according to this invention provides the appropriate strength, pressure resistance and water exclusion with a simple structure. Although some of its components resemble components of cables disclosed in the prior art, the cable has a novel structure. DE OLS No. 3201981 describes a heat resistant cable with a different and complicated tubular member. The fibres, which are contained in the bore of the tube, are enveloped in, for example, a silicone rubber. GB No. 2099179 describes cables with metal reinforcement. The cable includes glass fibres, each having at least one coating layer and twisted together at the desired pitch. The fibres are embedded, e.g. in a silicone resin. GB 1461540 describes a cable with non-orientated tube which includes a fibrous element in its wall. The fibres are contained in the bore of the tube.